There are currently in existence many commercial white karat gold compositions that have proven successful. Palladium is known to be an effective whitener of gold alloys derived from the Au/Pd/Ag system, and such alloys exhibit excellent workability and low hardness (see W. S. Rapson and T. Groenewald, "Gold Usage", Academic Press, 1978, p. 48). The drawbacks of these type alloys are their high cost and high melting temperatures due to their relatively high palladium content, with the high melting temperatures being an inconvenience for investment casters.
Nickel is also an excellent whitener of gold and, when also combined with copper, results in alloys having good mechanical properties, workability and casting characteristics (see A. S. McDonald and G. H. Sistare, "The Metallurgy of Some Carat Gold Jewelry Alloys", Gold Bulletin, 1978, Vol. 4, No. 4, p. 128). Nickel has been identified as allergenic, however, and its use in jewelry is currently regulated. Nickel in close contact with skin can cause nickel dermatitis, an allergic reaction. European studies have shown that about 10 % of the female population, predominantly those between the ages of 14 and 24, have a sensitivity to nickel (see "Focus: The Nickel Controversy in Europe", MJSA Publication, Vol. 4, No. 9, Sep. 1992). Among males, about 2% of the population is affected; this figure is expected to increase because increasing numbers of males are now having their ears pierced to wear earrings.
According to the Nickel Development Institute (see "Nickel and Nickel Alloy Articles That Come in Contact With the Skin", released by the Nickel Development Institute, July 1992), nickel in metallic form is not a sensitizing substance. Rather, sensitization and subsequent dermatitis are the result of a soluble corrosion product that occurs from the reaction of nickel with sweat that penetrates the skin. Accordingly, nickel-containing alloys that do not react with sweat will not cause dermatitis. For example, some stainless steels are non-allergenic, and it is most likely that a high karat gold alloy containing nickel will not react with sweat as well.
Also, transient contact with nickel is not harmful because there is insufficient time for a reaction with sweat. Thus, people can handle nickel-containing articles such as coins, tools, kitchenware, keys, etc. without experiencing nickel dermatitis. Sensitization can occur, however, when a significant exposure to nickel in soluble form takes place. Some dermatologists attribute the initial sensitization to the ear-piercing process, i.e., when a temporary stud that contains nickel is used during epithelization (the process of healing the wound). Here, nickel corrosion products can be present for a long time in the open wound and can cause sensitization.
Whether or not nickel containing materials are allergenic to individuals can be assessed through studies, now in progress in Europe, that involve Clinical Skin Patch Testing. In these tests, a patch containing the substance to be studied is directly applied on the skin for a certain time period. There are also tests, such as immersion in a synthetic perspiration solution, that are prescribed to determine the release of nickel. Preliminary information shows that surgical grade stainless steel (18-8) and high karat (18 Kt) nickel containing white golds appear to be nonallergenic, while brasses containing nickel and low karat (9-14 Kt) nickel containing white golds appear to cause an allergic reaction.
Thus, legislation in Europe is pending regarding articles such as earrings, bracelets, necklaces, rings, watch straps, etc. that come in direct and prolonged contact with the skin. For example, Denmark, since June 1989, has banned the sale of jewelry items that release nickel at a rate exceeding 0.5 micrograms per square centimeter per week. Germany prohibits the use of nickel on ear posts and, as of July 1993, requires a written warning label in articles that come in direct contact with the skin and release more than the above-stated amount. Sweden has set a limit of 0.05% nickel in alloys used for ear jewelry. Also, the European Community is working towards a common legislation on the nickel issue. Thus, there is a need to formulate compositions of white gold alloys that are essentially free of nickel.